The present invention relates to processes for applying catalyst or catalyst support coatings onto ceramic supports. More particularly, the invention relates to methods for coating ceramic substrates with catalyst coatings wherein a pre-coating or passivation step is used to improve the properties of the catalyzed substrates, by reducing catalyst and/or support coating diffusion into the fine pore and microcrack structure the substrates.
To address tightening diesel engine emissions regulations being adopted in the United States and Europe, recent attention has focused on basic improvements in the design and performance of ceramic wall-flow honeycomb filters for treating diesel exhaust gases. Among other improvements, design changes allowing for the use of catalyst coatings to control hydrocarbon and/or nitrogen oxide emissions are being implemented. The goal is to develop an improved high-temperature-resistant, high-thermal-shock-resistant, low cost honeycomb soot filter compatible with advanced emissions control catalyst technologies that can replace current high-cost and/or uncatalyzed particulate filters.
Among the filter designs being developed for this application are refractory ceramic oxide filters offering improved resistance to high exhaust temperatures encountered during decarbonizing filter regeneration cycles, as well as to the thermal shock conditions arising during rapid filter heat-up and cool-down in the course of startup and regeneration. Examples of advanced cordierite and aluminum titanate compositions and honeycomb filter designs being developed for these applications are disclosed in U.S. Pat. No. 6,541,407 and in co-pending, commonly assigned U.S. patent applications Ser. No. 60/400,248 filed Jul. 31, 2002, 10/209,684 filed Jul. 31, 2002, and Ser. No. 10/098,711 filed Mar. 14, 2002. Among other materials that are candidates for refractory, catalyst-compatible ceramic particulate filters are the refractory alkali zirconium phosphates as well as low-expansion alkali aluminosilicates such as beta-eucryptite and pollucite. Many of these same compositions, and other microcracked ceramic materials such as the calcium aluminates, are being considered for use as flow-through catalyst supports for the control of nitrogen oxide (NOx) emissions from automotive and diesel engines
Advanced aluminum titanate ceramics are among the most promising candidates for use in diesel exhaust filter applications, meeting or exceeding most specifications for high melting point, high thermal capacity, and low thermal expansion. However, one difficulty with these and other porous ceramics intended to function as particulate filters is the need to maintain both high gas permeability and a low coefficient of thermal expansion throughout the processes involved in depositing catalysts on the filter walls. A general requirement is that a low average linear coefficient of thermal expansion (CTE) for these filters should be maintained. Desirably, Increases in CTE resulting from the application of washcoats and catalyst should not exceed 10×10−7/° C. averaged over the range from 25–1000° C., and CTE values for the washcoated filters should not exceed 20×10−7/° C. over that temperature range, in order to preserve the thermal shock resistance of the filter. Further, gas permeabilities through the catalyzed filter should be sufficient to maintain pressure drops below 8 kPa at exhaust gas space velocities up to 150,000 hr−1 after filter regeneration to remove trapped particulates.
Problematically, significant increases in CTE and reductions in filter permeability frequently result from the application to these filters of the alumina or other washcoating materials customarily employed to support the required emissions control catalysts. Present understanding is that during the washcoating or catalyzing process, both wall porosity of the filter and the structural micro-cracks (crack widths of 0.1–3 microns) that are present in most of these ceramic materials are frequently filled with the washcoating material. The problem is most severe in the case of highly microcracked ceramics such as the aluminum titanates, particularly when the washcoating formulations contain materials of very fine particle size (e.g., particle diameters in the 0.02–0.1 μm range).
Microcracking is a significant contributor to the low CTEs exhibited by many of these materials, with crack closure during heating considerably moderating the dimensional increases that would otherwise occur. Thus the filling of these microcracks with washcoating constituents can result in some cases in much higher expansion coefficients, e.g., in the range of 40–50×10−7/° C., in the washcoated structures. At these CTE levels the risk of structural damage to the filters under the normal conditions of exhaust filter use is unacceptable.
One approach to the problem of washcoat microcrack filling that has been employed during the catalyzation of conventional flow-through catalyst substrates for gasoline engine emissions control has been the use of so-called passivating coatings. These are pre-coatings applied to the walls of the ceramic substrates prior to washcoating that can block the washcoating materials from intruding into the microcrack structure of the ceramic. U.S. Pat. No. 4,532,228 provides some examples of coating materials that can be carbonized or otherwise solidified to provide a washcoat barrier, and then removed after the washcoat has been laid down.
Unfortunately, these conventional coating approaches have so far not been effective to provide suitable passivating coatings for ceramic wall flow filters or advanced flow-through catalyst supports. One recurrent problem is that the known passivating coatings do not provide adequate protection against CTE increases in highly microcracked ceramics such as the aluminum titanates. Another problem is that such coatings are not very effective to guard against unacceptable reductions in ceramic wall gas permeability. The high gas permeability provided by the highly interconnecting wall porosity of advanced ceramic wall flow filters is of critical importance for engine exhaust filtration applications. Thus some means of simultaneously preserving the low thermal expansion coefficients and high gas permeability of advanced ceramic support or filter materials such as the aluminum titanates, even at high catalyst washcoat loadings, is required. Further the means selected cannot adversely affect the important catalyst support functions of the washcoat.